Semiconductor lasers have been widely used, but their most important characteristic, the threshold, has not been sufficiently investigated and remains highly controversial until now. Here, we illustrate this issue using an actual GaAs laser diode (LD). We analyze its electrical behavior, optical power, and electroluminescence (EL) spectrum near the threshold region, then propose that one should distinguish between laser transition points, including population inversion, lasing threshold, and saturation, respectively. Simple methods are provided to characterize the population inversion, the lasing threshold, and the saturation point. Additionally, we discuss the physical significance of these three points.
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